1. Field of the Invention
The present invention generally relates to a surveying instrument and a surveying method. The invention also relates to a computer program product causing the surveying instrument to perform the surveying method.
2. Brief Description of Related Art
A conventional surveying instrument comprises a base which can be mounted on a tripod and an optical system which is rotatably mounted relative to the base, wherein angle sensors are provided for measuring an orientation of the optical system relative to the base. The optical system may define an aiming direction which may coincide with an optical axis of the optical system or may be defined by a reticle provided in a beam path of the optical system. An operator may look through the optical system and orient the optical system such that the aiming direction is towards a target of interest. The operator may then perform a measurement of an angle of the direction to the target in a suitably chosen coordinate system of the surveying instrument.
The surveying instrument may have a tracking function wherein motors for controlling the orientation of the optical system relative to the base are controlled in dependence of an error signal which is indicative of a difference between the aiming direction of the optical system and the direction towards the target. For this purpose, the optical system may comprise a light source for generating a shaped beam of measuring light defining the aiming direction of the optical system, and a position sensitive radiation detector, such as a four quadrant detector, a position sensing detector or a CCD detector, receiving measuring light reflected from a reflecting target. The tracking system controls the motors such that an error signal derived from a detector signal of the radiation detector is minimized. Plural readings of angle values from the angle sensors are accumulated during a measuring period in which the tracking system maintains the orientation of the optical system such that the aiming direction of the optical system intersects with the target. The plural readings of angle values obtained during the measuring period are then averaged to produce an angle value indicative of the angle of the target in the coordinate system.
Turbulences in an air volume traversed by the beam of measuring light may cause deviations of a beam path of the beam of measuring light from a straight line, resulting in fluctuations of the tracking error signals. Due to the tracking control of the tracking system, the motors are continuously controlled in view of minimizing the error signal. This results in fluctuations of the angle values obtained during the measuring period. For obtaining a sufficient measuring accuracy it is necessary to accumulate a sufficient number of angle value readings which are averaged to determine the measuring result. Therefore, the measuring period must have a substantial duration, such as ten seconds or more, to ensure that a desired measuring accuracy is achieved in situations of high air turbulence. This results in a long duration of a measuring campaign in which measurements of many targets of interest are made.